[Webkit-unassigned] [Bug 141107] New: Converting Flushes and PhantomLocals to Phantoms requires an OSR availability analysis rather than just using the SetLocal's child

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=141107

            Bug ID: 141107
           Summary: Converting Flushes and PhantomLocals to Phantoms
                    requires an OSR availability analysis rather than just
                    using the SetLocal's child
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: fpizlo at apple.com

Consider that you have code like:

a: ...
b: MovHint(@a)
c: ValueRep(@a)
d: SetLocal(@c)
...
e: Flush(@d)

If the code between @d and @e doesn't observe the flushed value, we current convert this to:

a: ...
b: MovHint(@a)
c: ValueRep(@a)
d: SetLocal(@c)
...
e: Phantom(@c)

That's wrong, because @c is not MovHinted to anything. This causes @a to die and if we exit, we won't have a value for this local.

Our IR supports the notion that two logically equivalent but not identical values may be used for the MovHint and SetLocal.  Phantoms should use the values that MovHints used.  So, when strength-reducing a Flush or PhantomLocal to a Phantom we need to put the Phantom on the value that would have gotten MovHinted into that local.

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